The present invention relates to vehicle velocity sensors and more particularly to a multibeam radar velocity sensor that performs both Doppler shift and time-correlation measurements.
Existing Doppler aircraft radar velocity sensors suffer from a loss of functional operation over smooth water. This occurs because the angles of incidence and reflection of the radiated signal are the same over smooth water and no significant amount of RF energy is backscattered toward a radar receiver.
In the past, this problem has been approached by increasing RF power to compensate for the signal return loss over smooth water. However, solid state RF transmitters are peak power limited resulting in insufficient signal return for tracking most smooth water surfaces.
Velocity measurement through time correlation is an approach which has been demonstrated to provide good functional operation over smooth water; but requires considerable additional complexity to operate beyond limited drift angles (to xc2x145 degrees) and below 5 knots speed. However, use of the time correlator to only augment a Doppler velocity sensor permits the combined sensor to operate over the entire flight profile without the additional complexity required for the time correlation sensor. The reason that these two sensor techniques are complementary is because smooth water operation where the time-correlator is required occurs at low wind speeds (hence small drift angles), and at speeds greater than 5 knots because lower speeds occur at low altitude where the vehicle downwash roughens the water permitting Doppler operation.
Therefore, the basis of this invention is to add a minimal complexity time-correlator to a Doppler velocity sensor resulting in functional operation over all terrains.
The present invention offers a velocity sensor which performs over virtually all types of land and water terrains. The proposed system radiates and receives six beams of microwave energy in sequence, out of a common aperture, and utilizes a common signal processor for implementing both time correlation and Doppler measurements in order to measure the velocity components relative to the backscattering surface. If, for example, we consider a system on board an aircraft or other vehicle operating with six beams, the first four beams may be processed by measuring the Doppler shift in a return signal received by the antenna. From these four non-coplanar transmitted beams, the vehicle velocity can be computed. The last two beams are sequentially processed by two receive antennas which are mounted on the vehicle and separated by a fixed distance. A signal processor performs time-correlation measurements and makes use of the speckled nature of the power backscatter from the ground to measure the time delay between the power received by the receiving antennas. This measured time delay and the knowledge of the displacement between the receiving antennas allows the velocity of the vehicle to be computed. The velocity computed by Doppler and time correlation measurements may be averaged or weighed to achieve accurate measurement over widely varying terrain. For example, over most terrains and speeds, independent measurements of vehicle velocity are made from the Doppler shift measurements of the first four sequenced beams and the time correlation measurement of the last two beams. Over very smooth water, where the Doppler return signal of the transmitted beams 1-4 is sharply reduced due to the mirror-like nature of the water, the velocity measurement depends on the time-correlation measurement using the return signal from the last two beams which is increased in strength because it is directed perpendicularly to the backscattering surface.
The present invention lends itself to the construction of a single microstrip antenna capable of transmitting and receiving the multibeams for velocity measurements. Such a single antenna permits a combined time correlation and Doppler measurement to be made so that the present velocity sensor may operate over virtually all terrains and the entire vehicle speed range.